FIG. 28 is a perspective view showing a configuration of a conventional art optical pickup apparatus 1. FIG. 29 is a front view showing a hologram pattern 10. In a conventional art optical pickup apparatus, a light receiving element receives reflected light from a recording medium and thus, a positional shift with respect to an optical axis of an objective lens is detected (refer to, for instance, Japanese Unexamined Patent Publications JP-A 2002-92933 and JP-A 2002-237063). The conventional art optical pickup apparatus 1 shown in FIG. 29 comprises a light source 2, a grating lens 3, collimation lens 4, an objective lens 5, a hologram element 6, and light receiving elements 7a to 7h. 
Light from the light source 2 is split into one main beam 13a and two sub beams 13b and 13c by the grating lens 3 and then transmitted by the hologram element 6 and the collimation lens 7, and led to the objective lens 5. A first recording layer 9a of a recording medium 8 in a collected state is irradiated with the main beam 13a and respective sub beams 13b and 13c led to the objective lens 5. The main beam 13a and respective sub beams 13b and 13c reflected by the first recording layer 9a are transmitted by the objective lens 5 and the collimation lens 4, and led to the hologram element 6.
The hologram element 6 has the hologram pattern 10. The hologram pattern 10 has a first region 10a, a second region 10b, and a third region 10c. The first region 10a is one region in two half circles obtained by a parting line 11 passing through a center 10d of a circular region. The second region 10b is one region in two sectoral regions of the other half circle region, which sectoral regions obtained by a parting line 12 passing though the center 10d of the circular region and being perpendicular to the parting line 11. The third region 10c is the other region in the two sectoral regions.
FIG. 30 is a view for explaining light from the first recording layer 9a in a state where the objective lens 5 is located at a neutral position. FIG. 31 is a view for explaining light led to the respective light receiving elements 7a to 7h in a state where the objective lens 5 is located at the neutral position. FIG. 32 is a view for explaining one example of light from the first recording layer 9a in a state where the objective lens 5 is located at a position which is shifted in a radial direction A from the neutral position. FIG. 33 is a view for explaining one example of light led to the respective light receiving elements 7a to 7h in a state where the objective lens 5 is located at the position which is shifted in the radial direction A from the neutral position. FIG. 34 is a view for explaining another example of light from the recording medium 8 in a state where the objective lens 5 is located at the position which is shifted in the radial direction A from the neutral position. FIG. 35 is a view for explaining another example of the light led to the respective light receiving elements 7a to 7h in a state where the objective lens 5 is located at the position which is shifted in the radial direction A from the neutral position. When the objective lens 5 is located at the neutral position, the main beam 13a from the first recording layer 9a has an optical axis thereof passing through the center 10d of the hologram pattern 10, thereby enters the hologram element 6. At the time, the main beam 13a and respective sub beams 13b and 13c from the first recording layer 9a enter the second region 13b and the third region 13c at the same ratio, respectively.
When the objective lens 5 is disposed at a position which is shifted in the radial direction A from the neutral position, the main beam 13a from the first recording layer 9a has the optical axis thereof being displaced along the parting line 11. At this time, the main beam 13a from the first recording layer 9a enters either the second region 10b or the third region 10c in a biased state as shown in FIGS. 32 to 35. The main beam 13a and the respective sub beams 13b and 13c from the first recording layer 9a are diffracted by the respective first to third regions 10a to 10c. 
The light which has entered the first region 10a from the first recording layer 9a is diffracted and led to the light receiving elements 7a and 7b in order to detect a focus error signal. On the basis of a light receiving result by the light receiving elements 7a and 7b, the focus error signal is detected. Among the reflected lights which have entered the second region 10b from the first recording layer 9a, the main beam 13a is led to the light receiving element 7c, and the respective sub beams 13b and 13c are respectively led to the respective light receiving elements 7e and 7g. Among the reflected lights which have entered the third region 10c from the first recording layer 9a, the main beam 13a is led to the light receiving element 7d, and the respective sub beams 13b and 13c are respectively led to the respective light receiving elements 7f and 7h. On the basis of a light receiving result by the respective light receiving elements 7c, 7e and 7g corresponding to the second region 10b, and the light receiving result by the respective light receiving elements 7d, 7f and 7h corresponding to the third region 10c, the lens position signal is detected. By so doing, a positional shift of the objective lens 5 in the radial direction A from the neutral position can be obtained.
FIG. 36 is a view for explaining reflected lights from the first and second recording layers 9a and 9b. FIG. 37 is a view for explaining a reflected light from the second recording layer 9b in a state where the objective lens 5 is located at the neutral position. FIG. 38 is a view for explaining one example of a reflected light from the second recording layer 9b in a state where the objective lens 5 is located at a position which is shifted in the radial direction A from the neutral position. FIG. 39 is a view for explaining another example of a reflected light from the second recording layer 9b in a state where the objective lens 5 is located at the position which is shifted in the radial direction A from the neutral position. FIG. 40 is a graph showing a relation between a position of the objective lens 5 in the radial direction A, and an output value by the respective light receiving elements 7a to 7h. In the above-described optical pickup apparatus 1, in a case where light from the light source 2 is collected onto the first recording layer 9a, as shown by a virtual line 14 in FIGS. 28 and 36, a part of the light is transmitted by the first recording layer 9a and reflected by the second recording layer 9b. 
Since the second recording layer 9 is located at a position away from the objective lens 5 compared to the first recording layer 9a, the reflected light from the second recording layer 9b is reflected at a position which is further away than a focal length of the objective lens 5, and enters the hologram element 6 in a state of being focused by the objective lens 5 and the collimation lens 4. When diffracted by the hologram element 6, the reflected light from the second recording layer 9b enters a plurality of the light receiving elements with a larger spot size as shown by virtual lines 14a to 14c in FIGS. 37 to 39.
When the objective lens 5 is located at the neutral position, the output value shown by a lens position signal based on the reflected light from the first recording layer 9a. However, an output value shown by a lens position signal based on a reflected light from the second recording layer 9b does not become zero in the respective light receiving elements 7e to 7h for receiving the respective sub beams 13b and 13c because the sub beam 14b, for instance, enters the light receiving element 7g. 
Further, when the objective lens 5 is located at the position which is shifted from the neutral position, there is a case where the reflected light from the second recording layer 9b enters only either the second region 10b or the third region 10c of the hologram pattern 10. In this case, in a range of entering only either one of the second and third regions 10b and 10c, the reflected light from the second recording layer 9b enters the respective light receiving elements corresponding to the one region. At the time, the output value due to the receiving elements 7e to 7h for receiving sub beams becomes constant even when the objective lens 5 is displaced and therefore, an offset occurs in the output value shown by the lens position signal.
Moreover, among the reflected lights from the second recording layer 9b, when the main beam 14a enters the light receiving element for receiving the sub beam, an error component becomes larger because the main beam 14a has higher optical strength than the sub beam. A relation between an actual position of the objective lens 5 in the radial direction A, and an output value obtained by the sub beam 13 from the first recording layer 9a is shown by a graph which inclines largely due to the error component and which has a nonlinear property as shown by a solid line 15 in FIG. 40 due to the offset. This also gives a nonlinear property to a graph 16 showing a relation between the position of the objective lens 5 in the radial direction A, and the output value shown by the lens position signal. Since a nonlinear property which is different from an ideal graph 17 having a linear property is thus obtained, it is not possible to correctly obtain a position of the objective lens in relation to the neutral position with respect to the radial direction A.
Moreover, there is an optical pickup apparatus having a configuration that two light receiving elements are further provided at a position that the reflected light from the second recording layer 9b enters and a difference is secured, whereby offsetting the error component. In this optical pickup apparatus, there is only an effect on the focus error signal, and it is not possible to solve a state where the reflected light from the second recording layer 9b enters only either the second region 20b or the third region 10c. Accordingly, it is not possible to improve nonlinearity with respect to the lens position signal.
Moreover, just as the spot size of the reflected light from the second recording layer 9b becomes smaller in the receiving elements 7a to 7h, it can be thought of making the spot size of the reflected light from the second recording layer 9b larger in the hologram pattern 10. The spot size of the reflected light in the hologram element 6 and the respective light receiving elements 7a to 7h is determined by a distance between the respective layers 9a and 9b of the recording medium 8, and a lens magnification of optical system including the collimation lens 4 and the objective lens 5. The distance between the respective layers 9a and 9b of the recording medium 8 is previously determined by a standard. Moreover, the lens magnification of optical system is determined by a radiation angle of a light emitting element used as the light source 2. Thus, the distance between the respective layers 9a and, 9b of the recording medium 8 and the lens magnification of optical system cannot be easily changed for reasons of a configuration of an apparatus because, for instance, a trouble occurs in the apparatus when changed more than is necessary. In the conventional art optical pickup apparatus 1, it is thus not possible to realize a stable track servo since a correct lens position cannot be obtained.